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Lungfish
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Lungfish (also known as salamanderfish) are freshwater fish belonging to the Subclass Dipnoi. Lungfish are best-known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed internal skeleton. Today, they live only in Africa, South America, and Australia. While vicariance would suggest this represents an ancient distribution limited to the Mesozoic supercontinent Gondwana, the fossil record suggests that advanced lungfish had a widespread freshwater distribution and that the current distribution of modern lungfish species reflects extinction of many lineages following the breakup of Pangaea, Gondwana, and Laurasia.
lungfish demonstrate an uninterrupted cartilaginous notochord and an extensively developed palatal dentition.
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Encyclopedia
Lungfish (also known as salamanderfish) are freshwater fish belonging to the Subclass Dipnoi. Lungfish are best-known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed internal skeleton. Today, they live only in Africa, South America, and Australia. While vicariance would suggest this represents an ancient distribution limited to the Mesozoic supercontinent Gondwana, the fossil record suggests that advanced lungfish had a widespread freshwater distribution and that the current distribution of modern lungfish species reflects extinction of many lineages following the breakup of Pangaea, Gondwana, and Laurasia.
Anatomy and morphology
All lungfish demonstrate an uninterrupted cartilaginous notochord and an extensively developed palatal dentition. The lungfish is a true carnivore. Basal lungfish groups may retain marginal teeth and an ossified braincase, but derived lungfish groups, including all modern species, show a significant reduction in the marginal bones and a cartilaginous braincase. The bones of the skull roof in primitive lungfish are covered in a mineralized tissue called cosmine, but in post-Devonian lungfishes, the skull roof lies beneath the skin and the cosmine covering is lost. All modern lungfish show significant reductions and fusions of the bones of the skull roof, and the specific bones of the skull roof show no homology to the skull roof bones of ray-finned fishes or tetrapods. During the breeding season, the South American lungfish develops a pair of feathery appendages that are actually highly modified pelvic fins. It is thought these fins improve gas exchange around the fish's eggs in its nest.
The dentition of lungfish is different from that of any other vertebrate group. "Odontodes" on the palate and lower jaws develop in a series of rows to form a fan-shaped occlusion surface. These odontodes then wear to form a uniform crushing surface. In several groups, including the modern lepidosireniformes, these ridges have been modified to form occluding blades.
The modern lungfishes have a number of larval features, which suggest paedomorphosis. They also demonstrate the largest genome among the vertebrates.
Modern lungfish all have an elongate body with fleshy paired pectoral and pelvic fins and a single unpaired caudal fin replacing the dorsal, caudal, and anal fin of most fishes.
Lungs
All lungfish possess paired lungs which connect to the pharynx. While other species of fish can breath air via modified, vascularized swim bladders, these bladders are usually simple sacs, devoid complex internal structure. In contrast, the lungs of lungfish are subdivided into numerous smaller air sacs, greatly increasing surface area for improved gas exchange. Furthermore, while the aforementioned vascularized swim bladders have arisen independently in several lineages of fish, only in the lungfish are they homologous to the lungs of tetrapods.
Ecology and life history
Behavior
African and South American lungfish are capable of surviving seasonal drying out of their habitats by burrowing into mud and estivating throughout the dry season. Changes in physiology allow the lungfish to slow its metabolism to as little as 1/60th of the normal metabolic rate, and protein waste is converted from ammonia to less-toxic urea (normally, lungfish excrete nitrogenous waste as ammonia directly into the water).
Burrowing is seen in at least one group of fossil lungfish, the Gnathorhizidae. It has been proposed both that burrowing is plesiomorphic for lungfish as well as that gnathorhizids are directly ancestral to modern Lepidosireniformes, but it is possible that the similarity is simply due to convergent or parallel evolution.
Lungfish can be extremely long-lived. The Queensland lungfish at the Shedd Aquarium in Chicago has been part of the permanent live collection since 1933.
Taxonomy
The relationship of lungfishes to the rest of the bony fish is well-understood:
The relationships among lungfishes are significantly more difficult to resolve. While Devonian lungfish had enough bone in the skull to determine relationships, post-Devonian lungfish are represented entirely by skull roofs and teeth, as the rest of the skull is cartilaginous. Additionally, many of the taxa that have been identified may not be monophyletic. Current phylogenetic studies support the following relationships of major lungfish taxa:
Class Osteichthyes
Subclass Sarcopterygii
Order Dipnoi
,--†Family Diabolichthyidae
| ,--†Family Uranolophidae
| | __,--†Family Speonesydrionidae
'-|-| '--†Family Dipnorhynchidae
| ,--†Family Stomiahykidae
'----|___ ,--†Family Chirodipteridae
| '-|--†Family Holodontidae
|------†Family Dipteridae
| __,--†Family Fleurantiidae
'-| '--†Family Rhynchodipteridae
'--†Family Phaneropleuridae
| ,--†Family Ctenodontidae
'-| ,--†Family Sagenodontidae
'-|--†Family Gnathorhizidae
'--Order Ceratodontiformes
|--†family Asiatoceratodontidae
|--†Family Ptychoceratodontidae
|--Family Ceratodontidae
| '--†Genus Ceratodus
| '--†Genus Metaceratodus
'--Family Neoceratodontidae
| '--†Genus Mioceratodus
| '--Genus Neoceratodus - Queensland lungfish
'--Order Lepidosireniformes
'--Family Lepidosirenidae - South American lungfish
'--Family Protopteridae - African lungfish
See also
- Lepidogalaxias salamandroides
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